Diode pumped optical parametric oscillator

ABSTRACT

A diode pumped optical parametric oscillator is disclosed. The apparatus includes a MOPA laser positioned to pump a photorefractive BaTiO 3  crystal placed within a ring resonator formed by four mirrors. The interaction of the MOPA laser light incident on the BaTiO 3  crystal and with the light reflected within the cavity efficiently converts the MOPA pump beam to a single frequency beam of high beam quality. Once the pump beam exceeds a threshold power level, it interacts with a nonlinear periodically poled lithium niobate crystal received within the ring cavity. The lithium niobate crystal efficiently converts the pump beam to signal and idler beams of different wavelengths, providing an efficient, tunable laser.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

BACKGROUND OF THE INVENTION

The present invention relates generally to an apparatus for generatingoptical radiation, and more particularly, to a diode pumped opticalparametric oscillator involving three wave interaction to generatefrequency tunable laser beams.

The desirability of providing a high quality, tunable laser is wellknown. Many commercial applications such as environmental sensing of airpollutants and other remote sensing applications such as searching fornatural gas leaks, searching for gas and oil fields and spectroscopy ingeneral would be greatly benefited by a high quality, tunable laser. Asis known in the art, if atoms or molecules that absorb light at aspecific wavelength are illuminated with light of that wavelength, theycan be detected with an appropriate viewer. In this way, remote sensingfor pollutants, etc. by a choice of illumination wavelength is enabled.As can be appreciated, the greater the extent to which a laser istunable, the greater utility it will have in applications such as these.

Devices known as optical parametric oscillators are sometimes utilizedin the above applications because they operate to convert a first orpump laser beam into two, lower frequency beams commonly known as signaland idler beams. The signal and idler beams have wavelengths longer thanthat of the pump beam.

Optical parametric oscillators utilize a nonlinear process in a mediumto produce signal and idler beams. The wavelengths are determined by thephysical requirements that momentum and energy be conserved. These twoconservation laws result in the following equations for collinearphasematching:n_(pump)ω_(pump)=n_(signal)ω_(signal)+n_(idler)ω_(idler) ω_((pump))=ω_((signal))+ω_((idle))wherein ω represents frequency and n represents the refractive index, ameasure of the speed of light in the nonlinear material. Sincerefractive index is a function of frequency, crystal orientation, andbeam polarization, it is possible in some cases to simultaneouslyfulfill the requirements of both equations above.

A common characteristic of optical parametric oscillators is that theyutilize one or more nonlinear crystals placed within a reflectivecavity. The interaction of the pump beam with the nonlinear crystalgives rise to the generation of the signal and idler beams described inthe equations above. The nonlinear crystal can be angularly manipulatedwith respect to the pump beam to provide a tuning effect; other effectssuch as changing the temperature of the nonlinear crystal can also beused for tuning.

A recently developed configuration for an optical parametric oscillator,described by Bosenberg. et al., Continuous-wave singly resonant opticalparametric oscillator based on periodically poled LiNbO ₃, OpticsLetters, Vol. 21, No. 10, May 15, 1996, Optical Society of America,includes a Periodically Poled Lithium Niobate (PPLN) crystal as thenonlinear medium. This device represents a major advance over otheroptical parametric oscillator embodiments. High nonlinear gain, nobirefringent walkoff effects, and engineerable grating periodicity makecw optical parametric oscillators a practical reality for the firsttime. But this device uses a neodymium-doped yttrium aluminum garnet(Nd:YAG) crystal pumped by diode lasers to provide the high power pumpbeam. While this device represents an advancement over the art, it isnot without the need for improvement. More specifically, this device iscomplex because the diode laser light must be carefully coupled to theNd laser. Moreover, Nd lasers are somewhat inefficient, typicallyconverting only 30-40% of the diode pump power to Nd laser output power.The rest of the pump power becomes heat which must be dissipated.

While a direct substitution of the Nd:YAG laser with a high power diodelaser would overcome the above described efficiency problem, as well assimplify the device, high power diode lasers typically suffer from aninherent poor beam quality, rendering them unsuitable for opticalparametric oscillator applications.

A need exists therefore for an improved optical parametric oscillatorpumped by an improved high efficiency laser source. Such a laser sourcewould combine the desirable qualities of high power output, high beamquality for use within an optical parametric oscillator to provide highpower, high quality output.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide adiode pumped optical parametric oscillator overcoming the limitationsand disadvantages of the prior art.

It is another object of the present invention to provide a diode pumpedoptical parametric oscillator utilizing a simple high power diode laserbeam as the pump beam.

It is yet another object of the present invention to provide a diodepumped optical parametric oscillator utilizing a quasi phase matchedPPLN crystal as the nonlinear medium to provide diffraction limited highoutput power.

These and other objects of the invention will become apparent as thedescription of the representative embodiments proceeds.

In accordance with the foregoing principles and objects of theinvention, a diode pumped optical parametric oscillator is provided toconvert a pump laser beam at a first frequency to signal and idler beamsat different frequencies.

Advantageously and according to an important aspect of the presentinvention, the laser pump source is a master oscillator power amplifier(MOPA) semiconductor laser. The MOPA laser provides narrowband frequencyoutput at a watt or more output power.

The MOPA laser is positioned to pump a photorefractive BaTiO₃ crystalplaced within a ring resonator formed by four mirrors. When pumped bythe MOPA laser, the interaction of the light incident upon the BaTiO₃crystal from the MOPA laser and from the light circulated within thering resonator causes one light beam to grow at the expense of other,scattered beams due to the action of the beams within the refractiveindex grating created within the BaTiO₃ crystal. The net effect is thateven poor quality pump beam power is efficiently converted to a singlefrequency beam of very high quality. In this way, the poor qualitylimitation of typical high power diode lasers is overcome. This type oflaser device is described, for example, by J. Feinberg et al.,Phase-conjugate mirrors and resonators with photorefractive materials,Topics in Applied Physics—Vol. 62, Photorefractive Materials and theirApplications, II, P. Gunter and J.-P. Huignard, Eds. (Springer Verlag,1989).

In operation, the circulating pump beam, as continuously refined by theBaTiO₃ crystal, becomes single frequency, continuous wave, anddiffraction limited. Continued operation causes the pump beam power tobuild. Once the pump beam power exceeds a threshold level, the pump beaminteracts with a nonlinear periodically poled lithium niobate LiNbO₃(PPLN) crystal also placed within the ring resonator. The nonlinear PPLNcrystal within the four ring resonator mirrors forms an opticalparametric oscillator that interacts with the pump beam to output signaland idler beams of different wavelengths. The optical parametricoscillator operates in a nonlinear manner, and is constrained only bythe requirement that momentum and energy must be conserved.

Any of the mirrors in the ring resonator can be made partiallyreflective at a predetermined wavelength so as to allow the output oflight at that wavelength. Thus, the optical parametric oscillatorreceives light at a first, pump wavelength and outputs light at anotherwavelength, either that of the signal or idler beams. Advantageously,the BaTiO₃ containing ring resonator and the PPLN containing opticalparametric oscillator are coextensive such that each occupies the sameresonator and utilize the same set of four mirrors. This provides for asimplified, compact, highly efficient device.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing incorporated in and forming a part of thespecification, illustrates several aspects of the present invention andtogether with the description serves to explain the principles of theinvention. In the drawing:

FIG. 1 is a schematic illustration of the diode pumped opticalparametric oscillator of the present invention;

FIG. 2 is a schematic illustration of an alternative embodiment of thediode pumped optical parametric oscillator of the present invention;and,

FIG. 3 is a schematic illustration of another alternative embodiment ofthe diode pumped optical parametric oscillator of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1 showing the diode pumped optical parametricoscillator 10 of the present invention. The diode pumped opticalparametric oscillator 10 includes a source of coherent light 12.Advantageously, the source of coherent light 12 in the preferredembodiment is a master oscillator power amplifier semiconductor laser(MOPA) 14. The MOPA 14 is simple to operate, requiring no complexfeedback mechanism, and provides a narrowband frequency at a watt ormore output power.

The coherent light pump beam 16, generated by the MOPA 14, is directedinto a photorefractive element comprising a rubidium doped BaTiO₃crystal 18 located within a ring resonator cavity 20. As shown, the ringresonator cavity 20 is formed by four mirrors designated 22, 24, 26 and28 respectively. As shown, some (or all) of the mirrors 22-28, can becurved to provide tight focusing of the beams, in order to increase peakpower. During operation, as the pump beam 16 is directed into the BaTiO₃crystal 18, a refractive index grating is created within the crystal 18.The crystal 18 amplifies and refines the light received from the pumpbeam 16, and from the light reflected by the mirrors 22-28, generating ahigh quality single frequency beam 30 within the resonator cavity 20.

As operation continues, the single frequency beam 30 is furtheramplified within the resonator cavity 20 and the power level of the beam30 correspondingly increases. Once the power of the single frequencybeam 30 rises above a threshold level, it operates to pump a nonlinearperiodically poled lithium niobate LiNbO₃ (PPLN) crystal 32 receivedwithin the ring resonator cavity 20.

According to an important aspect of the present invention, the PPLNcrystal 32 operates in conjunction with the four mirrors 22, 24, 26 and28 to form an optical parametric oscillator 34 frequency conversionelement which is coextensive with the ring resonator cavity 20. Theoptical parametric oscillator 34, utilizes the quasi phase matchingnature of the PPLN crystal 32, in cooperation with the four mirrors 22,24, 26 and 28 to effectively convert the single frequency beam 30 into asignal beam and an idler beam.

The optical parametric oscillator 34 operates in a nonlinear manner togenerate the signal and idler beams. The wavelengths are determined bythe physical requirements that momentum and energy be conserved. Thesetwo conservation laws result in the following equations for collinearquasi-phasematching:n_(pump)ω_(pump)=n_(signal)ω_(signal)+n_(idler)ω_(idler)+k_(grating)cω_((pump))=ω_((signal))+ω_((idler))wherein ω represents frequency and n represents the refractive index, ameasure of the speed of light c in the nonlinear material andk_(grating) represents the effective momentum contributed by the poledgrating.

Accordingly, it can be seen that there is no explicit requirement thatthe signal and idler beams be related directly to the wavelength of thepump beam as long as they satisfy these equations. Thus, it is possibleto tune the laser output beam 36 over a wide range of wavelengths andfrequencies by adjusting the optical parametric oscillator 34. Forexample, the output beam 36 of the optical parametric oscillator 34 canbe effectively tuned by physically repositioning the PPLN crystal 32with respect to the angle of the single frequency beam 30. Or the PPLNcrystal 32 can have multiple grating periods poled into it and by simplytranslating the crystal relative to the pump beam 30 use a differentgrating period resulting in different output wavelengths. Or, the PPLNcrystal 32 can be heated in order to affect the internal poled spacingof the crystalline structure and hence change the output.

As shown in FIG. 1, mirror 22 is made partially reflective for thedesired output wavelength (signal or idler) thereby facilitating theemission of the output beam 36. The output beam 36 thus generated is ahigh power, continuous wave, diffraction limited beam. In this way,efficient direct pumping of an optical parametric oscillator by a diodelaser is facilitated. Advantageously, the diode pumped opticalparametric oscillator 10 requires no complex frequency feedback systemand no complex beam control.

Reference is now made to FIG. 2 showing an alternative embodiment of thediode pumped optical parametric oscillator 10. As shown, thisalternative embodiment is arranged such that the ring resonator cavity20 is physically separate from the optical parametric oscillator 34. Asshown, the ring resonator cavity 20 includes mirrors 22, 24, 26 and 28.The optical parametric oscillator 34 includes four mirrors designated38, 40, 42 and 44 respectively. As in the preferred embodiment, the ringresonator cavity 20 includes the BaTiO₃ crystal 18, and the opticalparametric oscillator 34 includes the PPLN crystal 32.

The operation of this alternative embodiment is quite similar to that ofthe preferred embodiment. The MOPA 14 outputs a pump beam 16 which isdirected into the BaTiO₃ crystal 18. The crystal 18 amplifies andrefines the light received from the pump beam 16, generating a highquality single frequency beam 30 within the resonator cavity 20. Themirror 22 is made partially reflective to the wavelength of the singlefrequency beam 30, facilitating emission of the output beam 36.

The single frequency beam 36 is directed into the optical parametricoscillator 34 wherein it is reflected by the mirrors 38, 40, 42, and 44,simultaneously passing through the PPLN crystal 32. The PPLN crystal andthe mirrors combine to form the desired frequency conversion elementwhereby the pump (single frequency beam 30) beam is split into signaland idler beams. The mirror 38 is made partially reflective as describedabove, to facilitate emission of the desired optical radiation.

Reference is made to FIG. 3 showing another alternative embodiment ofthe diode pumped optical parametric oscillator 10 of the presentinvention. This embodiment includes the same ring resonator cavity 20 asdescribed above, but differs in that the optical parametric oscillator34 in this embodiment is of the standing wave type utilizing a focusinglens 45 and two resonator mirrors, 46 and 48. Mirror 46 is transparentat the output beam (pump) 36 wavelength but highly reflective at thesignal and idler wavelengths. Similarly, mirror 48 is highlytransmissive of the desired output wavelength (signal or idler) andhighly reflective of the other wavelength. In this way emission ofoptical radiation at the desired wavelength is facilitated.

In summary, numerous benefits have been described from utilizing theprinciples of the present invention. The diode pumped optical parametricoscillator 10 of the present invention provides for an efficient,tunable laser utilizing a quasi-phase matched PPLN crystal pumped by aMOPA semiconductor diode laser.

The foregoing description of the preferred embodiment has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed.Obvious modifications or variations are possible in light of the aboveteachings. The embodiment was chosen and described to provide the bestillustration of the principles of the invention and its practicalapplication to thereby enable one of ordinary skill in the art toutilize the inventions in various embodiments and with variousmodifications as are suited to the particular scope of the invention asdetermined by the appended claims when interpreted in accordance withthe breadth to which they are fairly, legally and equitably entitled.

1. An apparatus for generating optical radiation, comprising: a sourceof coherent light; a ring resonator cavity for receiving, amplifying andconverting said coherent light to a high quality, single frequency beam,said ring resonator cavity including a photorefractive crystal; and, afrequency conversion element coextensive with said ring resonator cavitypositioned to receive said single frequency beam, said frequencyconversion element having a nonlinear medium located within forconverting said single frequency beam into signal and idler beams. 2.The apparatus of claim 1 wherein said ring resonator cavity is formed byfour mirrors.
 3. The apparatus of claim 1 wherein said photorefractivecrystal is BaTiO₃.
 4. The apparatus of claim 1 wherein said nonlinearmedium is periodically poled LiNbO₃.
 5. The apparatus of claim 1 whereinsaid source of coherent light is a diode laser.
 6. The apparatus ofclaim 5 wherein said diode laser is a master oscillator power amplifiersemiconductor laser.
 7. An apparatus for generating optical radiation,comprising: a source of coherent light; a ring resonator cavity forreceiving, amplifying and converting said coherent light to a highquality, single frequency beam, said ring resonator cavity including aphotorefractive crystal; and, a frequency conversion element positionedto receive said single frequency beam, said frequency conversion elementhaving a nonlinear medium located within for converting said singlefrequency beam into signal and idler beams.
 8. The apparatus of claim 7wherein said ring resonator cavity is formed by four mirrors.
 9. Theapparatus of claim 8 wherein said photorefractive crystal is BaTiO₃. 10.The apparatus of claim 7 wherein said nonlinear medium is periodicallypoled LiNbO₃.
 11. The apparatus of claim 7 wherein said source ofcoherent light is a diode laser.
 12. The apparatus of claim 11 whereinsaid diode laser is a master oscillator power amplifier semiconductorlaser.
 13. The apparatus of claim 7 wherein said frequency conversionelement is a standing wave cavity defined by a focusing lens and twomirrors.